Light-emitting diodes, commonly called LEDs, continue to increase in popularity as a light source for use in many and diverse applications. The demand for LEDs has grown rapidly, especially in the last five years. LEDs are being used as light sources in numerous applications due to their many advantages over conventional light sources. LEDs generally consume significantly less power than incandescent and other light sources, require a low voltage to operate, are resistant to mechanical shock, require low maintenance, and generate minimal heat when operating. As a result, LEDs are displacing incandescent and other light sources in many uses and have found applications, for instance, as traffic signals, large area displays, interior and exterior lighting, cellular telephone displays, digital clock displays, displays for consumer appliances, flashlights, and the like.
LEDs generally include a light-emitting diode mounted on a substrate that is electrically connected to a lead frame. The lead frame typically includes two terminals for connecting the LED to a power source. The light-emitting diode is a semiconductor device fabricated similar to the manner in which integrated circuits are produced. For instance, the light-emitting diode can be made from several layers of material that are sequentially deposited on a semiconductor substrate. The light-emitting diode within the semiconductor material includes an n-type material separated from a p-type material by an active layer. When a voltage is applied to the diode, positive charges or “holes” from the p-type material move towards the active layer while the negative charges or electrons from the n-type material also move towards the active layer in an opposite direction which produces light. In particular, the moving electrons release energy in the form of photons. Thus, one significant advantage of LEDs is that the devices produce light without a filament that will burn out over time. Thus, LEDs last a relatively long time, can be made to be very compact, and are very durable. Further, since a filament is not heated in order to produce light, LEDs are also very energy efficient.
After a light-emitting diode is fabricated, the semiconductor chip can be mounted adjacent to a reflector and connected to a lead frame. The lead frame can include an anode terminal and a cathode terminal for applying power to the assembly. In certain embodiments, the LED element located within the reflector can be sealed by a translucent or transparent resin. The transparent or translucent resin may serve as a lens for further enhancing the light that is emitted.
The reflector for the LED can also serve as the housing for the LED and is typically made from a molded polymeric resin. For example, the polymeric resin can be injection molded to form the housing and reflector. In one embodiment, the polymeric resin is injection molded over a lead frame for integrating the lead frame into the LED assembly.
The molded polymer resin used to form the reflector preferably possesses a particular combination of characteristics and properties. For instance, the polymer resin should be well suited to reflecting light at the wavelength at which the LED operates. Many LEDs, for instance, are designed to emit a white light. Thus, the polymer resin used to form the reflector should reflect a significant amount of light in the visible light region and particularly should reflect a significant percentage of light in the blue light wavelength range. Reflecting light in the blue wavelength range, for instance, has been found to significantly enhance the brightness of the LED, since LEDs that emit a white light emit a significant amount of light in the blue wavelength range. Increasing the reflectance of the reflector as high as possible minimizes loss of light when the LED is being operated.
The polymer resin used to form the reflector should also possess a high whiteness index. The whiteness index of the reflector indicates how well the reflector can reflect light over the entire visible light wavelength range (from about 400 nm to about 700 nm). In general, the higher the whiteness index of the material, the higher the reflectance of the material. A material possessing a white index value of 100, for instance, is considered a substantially perfect reflecting diffuser.
In addition to having excellent reflectance properties, the polymer resin used to form the reflector should also have good melt flow properties during injection molding of the parts. For instance, many LED structures are relatively small having dimensions that at times can be less than 1 millimeter. Reflectors can also have relatively complex shapes depending upon the particular application and the geometries of the lead plate in the LED. Thus, when the polymer resin is heated, the polymer should have sufficient flow properties in order to uniformly and repeatedly fill the interstices of the mold. The polymer resin should also have a stable viscosity that does not fluctuate during processing.
In addition to the above, the polymer resin used to form the reflector should have sufficient heat resistance including long term aging stability when either being soldered onto an adjacent part or when exposed to the operating temperatures of the LED. Many LED assemblies, for instance, are attached to circuit boards and other substrates using reflow oven welding processes that operate at temperatures up to about 260° C. The polymer resin should have good heat resistance properties to the reflow process and should not blister or otherwise deteriorate when subjected to the welding conditions.
During use, the LED also generates heat which is absorbed by the reflector. In the past few years, the amount of heat generated by the LED has increased as the LED element power has increased. When subjected to heat during welding and/or heat during use, the reflectance properties of the polymer resin should not deteriorate. In the past, for instance, exposure to high temperatures and/or repeated heating and cooling during use have caused polymer resins to yellow. Yellowing causes the whiteness index of the resin to lower. Yellowing is especially a problem for LEDs that emit blue light since yellow surfaces have a tendency to absorb light in the blue wavelength range.
In addition to the above, the reflector is generally a thin small part and requires satisfactory mechanical strength. Thus, reflectors should also have sufficient impact strength to avoid breakage during assembly of the LED and during use of the LED.
In U.S. Patent Publication No. 2007/0213458 entitled “Light-Emitting Diode Assembly Housing Comprising Poly(cyclohexanedimethanol terephthalate) Compositions”, a reflector for an LED is disclosed that is made from a poly(cyclohexanedimethanol terephthalate) (hereinafter “PCT”) composition. The '458 application, which is incorporated herein by reference, has made great advances in design and function of LEDs. The present disclosure, however, is directed to further improvements. In particular, the present disclosure is generally directed to a reflector for an LED made from a PCT polymer composition that has improved viscosity characteristics and/or reflectance characteristics, including better resistance to yellowing after thermal aging.